The use of carbonaceous materials such as coal as a fuel for the production of electrical power is widely accepted in most industrial countries. A number of the known processes involve the gasification of solid carbonaceous material with air at elevated temperatures often in the presence of steam to partially combust the carbonaceous material to produce a mixture of combustible gases. In combustion or gasification processes all of the moisture in the coal has to be dried either prior to the process in a separate drier or during the process. As drying is an energy consuming process, the efficiency of a power plant can be significantly improved by the choice of the drying process.
In recent years, a number of more advanced technologies have been developed to improve the thermal efficiency of coal fuelled power stations, reduce emission levels of NO.sub.x, SO.sub.x and CO.sub.2 as well as reduce the overall cost of electricity. Most of the advanced technologies that have been reported have been trialled and practiced on low moisture content carbonaceous materials such as high-rank coals which have a moisture content typically less than 10%.
Countries such as Australia, Germany, USA, China, Indonesia and India have large deposits of low-rank coals having high moisture contents which typically may exceed 60 wt % wet basis. One of the specific issues to be considered in the utilisation of high moisture carbonaceous materials for power generation is the need to incorporate a coal drying process. Other differences between high-rank coals and low-rank coals must also be considered in the design of a power generation system. Low rank coals generally have a higher alkali metal in ash content which affects fouling and corrosion and have a higher reactivity associated with a higher oxygen level and porosity, and the catalytic effect of the alkali.
In a conventional low rank coal fuelled boiler plant, raw coal is dried in direct contact with hot flue gas aspirated from the furnace by fan or beater mills. The dried pulverised coal is directed to the furnace (via the burners) entrained in the flue gas which has been cooled as a result of the drying.
A similar process has been proposed in Australian Paten No 661176 in the name of the State Electricity Commission of Victoria referred to as an Integrated Drying and Gasification Combined Cycle (IDGCC) process. In this proposal, the coal is dried in the hot fuel gas leaving the gasifier. After drying, the coal is separated from the gas in a device such as a cyclone for feeding to the gasifier. The cooled fuel gas, together with evaporated moisture, is directed via a hot gas filter to the gas turbine.
While this process has the possibility of increasing power output, due in part to the moisture evaporated from the coal being passed through the gas turbine, it operates on the assumption that the gasification of the coal is virtually 100% complete.
Another drying process which is similar in principle, has been developed as part of the IVOSDIG gasification process for wet fuels (S Hulkkonen, M Raiko, MAijala, Ref. High efficiency power plant processes for moist fuels. IGTI-Vol 6, 1991 ASME Cogen-Turbo Book No. 100313-1991). In this process superheated steam at a pressure slightly greater than the gasifier pressure is used to dry the fuel in a direct contact drier. The energy for drying is obtained from the cooling of the superheated drying steam. After drying the fuel is separated in a cyclone and fed to the gasifier.
After the cyclone, the part of the stream flow corresponding to the moisture evaporated from the fuel is injected into the gas stream from the gasifier for expansion through the gas turbine. The flow corresponding to the heating steam is recirculated through a super heater by a fan for reheating.
Other less integrated drying methods may also be used. In these the coal is dried at or near atmospheric pressure and then injected into the pressurised gasifier or combustor of the advanced power generation plant via lock hoppers or the like. The most commonly used driers for drying brown coal in Victoria and Europe are the rotary drum type, which are used, for example, in the making of briquettes. In these driers, coal is fed at atmospheric pressure through tubes contained in a drum or shell and the heat of drying is supplied by steam condensing on the outside of the tubes. The moisture evaporated from the coal is carried through the tubes in a low velocity air stream.
More recently another indirect steam drying process has been developed involving a steam fluidised bed drier (SFBD). In a SFBD, crushed coal is dried in a fluid bed at a pressure slightly greater than atmospheric pressure using a steam as the fluidising medium. The water evaporated from the coal adds to the fluidising steam so that the coal is contained entirely in an inert steam environment. The heat for drying is supplied to the bed through tubes by steam condensing at a pressure of typically about 5 bar.
An advanced technology for power generation from coal to have achieved essentially a commercial status is the combined cycle technology of pressurised fluid bed combustion (PFBC). The potential for further development of PFBC has been widely recognised. The performance of PFBC is limited by the maximum temperature allowed in the fluid bed (about 860.degree. C.) which in turn controls the temperature of the gas to be expanded through the gas turbine. The gas turbine output from a PFBC system as a result is limited to about 20% of the total electrical output.
The A-PFBC (Advanced Pressurised Fluid Bed Combustion) system primarily was intended to overcome the gas turbine inlet temperature constraint of PFBC. By combining the PFBC with a gasifier, the gas turbine inlet temperature can be raised to the maximum allowable temperature which is currently around 1260.degree. C. in modern large gas turbines. This increases the power generated from the gas turbine (Brayton cycle) to about 50% of the total electrical output. In an A-PFBC system, the coal is reacted in two stages and is referred to as a hybrid system. The coal is first partially gasified in a carboniser to produce a stream of low calorific value fuel gas and a stream of char. The char is then burned separately in a pressurised fluid bed boiler to generate steam. The low specific energy gas from the carboniser is mixed with the combustion products of the char at a temperature of about 860.degree. C. and burned with air in a topping combustor. Both the fuel gas and the flue gas are cleaned prior to entering the topping combustor. The resulting flue gas, which is controlled by excess air so as not to exceed the allowable maximum turbine inlet temperature of currently about 1260.degree. C., is then expanded through the gas turbine. The air supplied to the carboniser, PFBC and topping combustor is extracted from the gas turbine compressor. A booster compressor is used only for the air supplied to the carboniser. Steam is also generated in the heat recovery boiler of the gas turbine and this together with the steam from the PFBC is used to drive a steam turbine.
As discussed above, all of the advanced technologies which are presently being developed are being trialled and practiced with high-rank coals which by nature have a low moisture content. One method for adapting technologies which require low moisture coal to use with high moisture carbonaceous materials is to add a separate pre-drying step.